Why does Curley threaten Lennie?

Curley threatens Lennie because he thinks he is laughing at him, when in reality, Lennie is smiling at the thought of the ranch..... and the rabbits.

His eyes slipped on past and lighted on Lennie; and Lennie was still smiling with delight at the memory of the ranch.

Curley stepped over to Lennie like a terrier. “What the hell you laughin’ at?”

Lennie looked blankly at him. “Huh?”

Then Curley’s rage exploded. “Come on, ya big bastard. Get up on your feet. No big son-of-a-bitch is gonna laugh at me. I’ll show ya who’s yella.”